1. Field of the Invention
The invention relates to a guide bushing used to maintain alignment of a rotary shaft and in particular to a guide bushing for use as a deck bushing in a mining blast hole drill.
2. Description of the Prior Art
In surface mining operations, there is a requirement to use rotary drills for drilling blast holes. A drilling arrangement for this purpose usually is in the form of a self-propelled unit having a mast or tower, and power means for rotating a drill pipe and feeding it into the ground. The drill pipe has a suitable rotary cutting bit on its lower end, such as a tri-cone rotary bit.
In a standard arrangement, the drill pipe extends downwardly from a top drive unit and through a deck bushing located on the deck of the drill and then into the ground. The purpose of the bushing is to prevent wear of the deck and keep the drill pipe as straight as possible. There is sufficient clearance between the outside diameter of the drill pipe and the inside diameter of the deck bushing such that the drill pipe and bushing are normally not in contact. When the drill pipe starts to run off laterally, as it frequently does during rock drilling operations, it comes into contact with the bushing.
Prior art bushings consisted of a steel sleeve that was prevented from rotating by lock lugs on the deck of the drilling machine. When the drill pipe came into contact with this type of bushing, rapid wear of both the drill pipe and bushing occurred, because of the radial stress present. Canadian Pat. No. 963,889 issued on May 4, 1975 to Donald Maclean, describes a guide or deck bushing that partially overcomes the short service life problem of the fixed sleeve bushing. This guide bushing includes a rotatable inner sleeve supported by one or more rows of ball bearings. By providing a rotatable inner sleeve in the deck bushing the service life of the bushing is increased substantially. Unfortunately, deck bushings of the Maclean type suffer from premature bearing wear. The problem occurs because during a drilling operation, when the drill pipe comes into contact with the deck bushing, there is not only a force in the radial direction, but there is also a substantial friction force in the axial direction between the drill pipe and the inside diameter of the inner sleeve, due to axial vibration of the drill pipe. Consequently, since the inner sleeve is supported by the ball bearings and prevented from moving in an axial direction, very high contact stresses are generated between the ball bearings and the grooves they are located in, which leads to premature bearing failure.